This invention relates generally to jet engines for aircraft, and more particularly to an apparatus and method for modifying a jet engine to be carried in an inoperative condition externally on an aircraft so as to reduce aerodynamic drag, buffeting, and engine oscillation.
Certain multi-engine jet transport aircraft are designed to permit an extra inoperative engine to be carried externally on the aircraft. This design feature permits an engine to be transported rapidly from place to place without sacrificing internal cargo capacity or in the event that the engine cannot be fitted into the internal cargo compartments. On a typical four-engine jet transport having the engines suspended below the wings on pylons, a spare engine is usually suspended below the wind just inboard of one of the two inboard engines by means of a spare engine pylon or strut. Normally, attachment points are provided on the undersurface of the wing to permit the temporary attachment of a spare engine pylon.
While convenient, the external carrying of an unmodified inoperative engine may create certain aerodynamic and structural dynamic problems. In flight, ram airflow is free to pass through the engine but the flow rate through the engine is much lower than it would be if the engine were in operation. As a result, a significant portion of the flow that would normally pass through the engine is forced to spill around the outside of the inlet, causing unusually high flow velocities and negative pressure around the outside of the inlet lip. These velocity and pressure irregularities have been found to be very sensitive to slight changes in the direction and Mach number of oncoming flow. At higher Mach numbers this nonsymmetrical unsteady flow acting on the spare engine nacelle may induce undamped lateral oscillations of the engine and may require an unacceptable restriction of aircraft Mach number. Also, spillage of flow also causes flow separation over the nacelle and a significant increase in aerodynamic drag and buffeting on the aircraft. This situation can be further aggravated if ice deposits form on the internal parts of the spare engine in flight and cause further blockage.
Various solutions have been proposed to minimize the problems created by an externally carried inoperative engine. Certain temporary modifications of the engine are currently known and used to minimize internal flow blockage. First, the fan blades are removed from the engine. Also, the engine core air inlet is covered with an ice deflector dome to prevent ice buildup from completely blocking the fan exit guide vanes. The dome is normally sized to provide a passage area at the location of the fan face approximately equal to the fan nozzle exit area. Also, some drag reduction may be achieved with certain engine configurations by placing a conical-shaped fairing over the core exhaust nozzle. While these three modifications have resulted in some improvement, it has been found that unacceptable air speed and Mach number limitations still must be placed on the aircraft to prevent undamped lateral oscillations of the spare engine, excessive drag and buffeting.
It has also been proposed that the entire air inlet of the spare engine be covered with a streamlined dome to direct air approaching the engine smoothly around the outside of the nacelle. A variation of such a dome is the apparatus described in U.S. Pat. No. 3,253,404 to Eduard Tonnies. While such a dome might eliminate some of the problems caused by air spillage from the engine, it causes excessive drag and buffeting by not allowing airflow to pass through the inlet and fill in the large void aft of the fan exit. Further, it has been found that it does not necessarily eliminate engine oscillations.
Various other devices for modifying the shape of the air inlet of a jet engine are found in the prior art. One such device is illustrated in U.S. Pat. No. 3,532,129 to Peter Arthur Ward. Ward discloses an air inlet for a gas turbine engine having a number of flaps mounted between the nacelle walls which can be extended forward so as to reduce the cross-sectional area of the air inlet. It appears that the objective of his invention is to block the forward propagation of noise from the engine compressor by creating choked flow at the inlet. Another device which might be employed to reduce flow spillage from an inoperative engine is described in U.S. Pat. No. 3,763,874 to Geoffrey Light Wilde and Leonard John Rodgers. In this patent the inventors disclose an air inlet lip which includes a number of translating, rotating segments which permit the changing of the effective radius of curvature of the lip. While both of these devices might be employed to solve the aerodynamic problems mentioned above, it appears that each would cause a significant and permanent increase in the weight and complexity of the nacelle structure as compared to the present invention.
Also relevant to the present invention is the variable area air intake disclosed in U.S. Pat. No. 2,931,167 to Rene Leduc. The invention disclosed therein relates to an air intake for a supersonic aircraft in which a conical center body cooperates with an axially movable outer nacelle to form a variable area annular air inlet. This invention would not be useful in solving the problems herein addressed because the outer nacelle, while movable, does not change in shape and because the engines to which the present invention relate do not have a conical center body of the type referred to by Leduc.
An object of this invention is to provide for an inlet extension which can be quickly and temprarily attached to the normal operating inlet of a jet aircraft engine to be carried externally in an inoperative condition on an aircraft which will significantly reduce local inlet lip velocities and negative pressure and delay the onset of undamped lateral oscillations of the engine. Another object of this invention is to provide for an inlet extension which will also substantially reduce flow separation and aerodynamic drag caused by flow spillage from such an inoperative engine. Another object of this invention is to provide for an improved method for modifying a jet aircraft engine which is to be carried externally on an aircraft in an inoperative condition so as to eliminate the necessity for unreasonable restrictions on airspeed and Mach number.